Calcium phosphate-induced fusion of human erythrocyte ghosts monitored by dilution of a membrane bound fluorescence probe.

Calcium-phosphate induced fusion of human erythrocytes was investigated using an assay based on the relief of the fluorescence selfquenching of a fluorescent amphiphile incorporated into ghost membranes as it occurs when labeled membranes are fused with unlabeled membranes. Measuring ghost fusion up to Ca(2+)-concentrations of 4 mM in the presence of 10 mM phosphate, both an acceleration of the fusion process and an enhancement of the fusion extent with increasing amounts of calcium were observed. Fusion takes place even in the case when calcium-phosphate complexes were formed in the suspension medium prior to the addition of ghosts. These results are in contrast to previous observations, obtained by an internal aqueous content mixing assay, where fusion of ghosts was inhibited at Ca(2+)-concentrations greater than 1.75 mM, and preformed calcium-phosphate complexes were not able to induce fusion. The results point to the necessity of utilizing content mixing assays in conjunction with the lipid dilution assay to get a more detailed insight in membrane fusion mechanism(s).